This invention is related to an element for a sootblower and in particular to a ceramic composite element.
Sootblowers are used to project a stream of blowing medium such as water, air, or steam against heat transfer surfaces within large scale boilers, such as a recovery boiler, to remove slag and ash encrustations from the boiler surfaces. The blowing medium impact produces mechanical and thermal shock which causes adhering layers of these encrustations to be removed.
One general category of sootblowers is known as the long retracting type. These devices have a retractable lance tube which is periodically advanced into the boiler interior and simultaneously rotated such that one or more nozzles along the length of the lance tube project jets of the blowing medium against the boiler surfaces. The lance tube is withdrawn from the boiler upon completion of the blowing process.
A retracting type sootblower is used in applications where the internal temperature of the boiler and/or the corrosive environment within the boiler interior is such that a metallic manifold pipe, or element, cannot endure permanent installation within the boiler. A significant disadvantage with the long retracting type of sootblower is that it is necessary to provide a considerable clearance area surrounding the boiler for the lance tube and to provide a drive mechanism to insert the lance tube into the boiler. In lower temperature and less corrosive environments, a metallic element can be permanently positioned within the boiler and periodically activated to rotate and project jets of blowing medium against the boiler surfaces. However, the life of the metallic element is relatively short, requiring frequent replacement.
Accordingly, it is an object of the present invention to provide a sootblower element that can be permanently installed in a high temperature and highly corrosive environment that previously required a retractable type metallic lance tube.
It is a further objective to provide a permanently installed element with a longer life span than permanently installed metallic elements.
It is a feature of the present invention to provide an element of ceramic tubing that is resistant to the high temperature and corrosive environment found within a boiler, with the result that the element can be permanently installed within the boiler. This eliminates the need for a clearance area surrounding the boiler and eliminates the need for a lance tube drive mechanism. The ceramic element can also replace permanently installed metallic elements used in less corrosive environments to provide a element with a longer life.
Monolithic ceramics are inorganic, nonmetallic materials, consisting of oxides (e.g., alumina and zirconia) or nonoxides (e.g., silicon nitride and boron carbide). In the form of pottery and brick, monolithics have been known to man for thousands of years. With the advent of modern science, it has become possible to fabricate new types of ceramics and they have found application in numerous areas because of their high strength, and good resistance to abrasion, erosion, and corrosion. Despite these desirable characteristics, monolithics have one basic flaw, they fail catastrophically with minimum reliability. For example, a coffee mug shatters when it is dropped due to its brittle fracture structure. That is, a crack is able to propagate completely through the ceramic. Because they fail in brittle fracture, monolithic ceramics may not be suitable for sootblower elements.
Ceramics composites consist of a multiphase ceramic, where one phase represents a reinforcement consisting of particulates, whiskers, or fibers. Fibers can be continuous or discontinuous. Ceramic composites have been shown to have higher toughness and impact resistance, and greater strain to failure than monolithic materials. Other properties (strength, thermal stability, corrosion resistance) potentially rival monolithics. When a continuous fiber composite is highly stressed, cracks form in the matrix surrounding the fibers, however, the fibers themselves do not fracture. Hence, the fibers are able to divert and diffuse the cracks that do form, thus preventing complete failure until a much higher level of stress is reached. This phenomenon is known as nonbrittle fracture, and is an additional benefit of continuous fiber composites.
Desirable attributes of a ceramic composite for use as a sootblower element include: fiber structure that can be oriented to optimally resist anticiated loads; thermal shock resistance; ability to be bonded to other ceramics or metals; machinability; continuous fiber structure to impart nonbrittle failure; resistance to corrodents found within a boiler; abrasion resistance; and low permeability.
Thermal shock resistance is highly desirable due to the stresses induced by the blowing medium. The blowing medium will be hundreds of degrees cooler than the normal temperature of the sootblower element. Therefore, when the blowing medium is injected through the element, the interior surface of the element will be rapidly cooled which will create severe tensile stresses within the element wall.
The sootblower element of the present invention may be constructed from one or more tubular ceramic composite segments that are manufactured by filament winding a ceramic fiber around a cylindrical mandrel to create a porous, fibrous preform. Alternatively, the fiber preforms can be fabricated using a braided or weaving processes. Filament winding, braiding, or weaving are the preferred methods of preform manufacture as the ceramic fiber can be optimally oriented to resist anticipated loads. The preform is subsequently processed using a sol-gel technique to create a dense matrix within the preform. Coatings may be applied to the surface of the tube to enhance its corrosion resistance. One or more segments can be connected together utilizing a ceramic coupling member that is also manufactured using the same preform fabrication and sol-gel process.
Ceramic nozzles are inserted into the lance tube element to form the nozzles from which jets of the blowing medium are projected. The nozzles may be produced using traditional ceramic fabrication techniques, or they may consist of composites created using a preform fabrication and sol-gel process.
Further objects, features, and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in conjunction with the accompanying drawings.